Gas chromatographic (GC) techniques were employed for identification of micro-organisms in clinical specimens by: 1) detecting one or more characteristic products of microorganisms in vitro and in vivo, 2) comparing metabolic profiles ("fingerprints") of organisms in infected and uninfected specimens, and 3) analysing products of microbial isolates after a brief incubation in an enriched broth culture medium. The GC procedures involving fractionation, solvent extraction, derivative preparation and sample injection were automated for rapid analyses. Chromatographic data obtained by four columns and four detectors, operating simultaneously, were analysed by an on-line integrator-tape recorder system. The chromatographic patterns were compared either manually or by using a digital computer. Gas chromatograms of serum samples from rats infected with a single strain from the general Salmonella, Pseudomonas, Escherichia, Proteus, Serratia, Haemophilus, Streptococcus, Staphylococcus, or Diplococcus yielded 2 to 8 characteristic peaks which were readily discernible. Characteristic GC profiles of human infections were obtained for bacterial infections due to Mycobacteria, Pneumococci, Staphylococci or certain Gram negative bacilli. By analysis of chromatograms of 300 coded sera from infected patients, the infective agents were rapidly identified with 97% accuracy. Gas chromatographic data of organic acid products from 200 different bacterial strains were digitalized and stored in a computer for rapid analysis. The identification of unknown bacterial isolates within these standards was achieved in approximately four hours. The GC analysis of sera or tissue samples from mice infected with Vesicular Stomatitis virus, Influenza virus or Herpes virus showed a characteristic metabolic profile for each type of infectious agent tested. From these studies, it seems that gas chromatography is a potentially useful tool for rapid automated identification of microorganisms in clinical specimens.